The present invention relates to a method of forming lips on thermoplastic containers, such as disposable cups, food packages, etc., utilizing a lip rolling machine.
Lip-rolling machines are used to produce a rolled rim on plastic containers. After trimming of the container from a thermoplastic sheet, the open end of the container has a rather sharp edge with an outward flare. The reason for rolling this edge is at least two-fold. First, drinking from a cup made in this fashion is a less than suitable undertaking for the user. Second, it is difficult to manufacture a lid which will fit about the flared edge and still maintain a suitable seal against the container rim.
Conventional lip rolling machines utilize an open ended, clam-shell type electric heating oven to heat and form lips on horizontally-nested thermoplastic containers, such as cups, as they are advanced off a production line. U.S. Pat. No. 6,093,010, issued Jul. 25, 2000, to Lamson, discloses a typical thermoforming process, including a clam shell oven. The oven often generates heat to cause internal surface temperatures of the oven to reach in excess of 500xc2x0 F. with the heaters themselves getting up to 1200xc2x0 F. However, when the production line is stoppedxe2x80x94due, for example, to a thermoforming machine malfunctionxe2x80x94a length of cups remain in the hot open oven. If this length of cups (approximately two feet of nested cups) is not removed from the oven when the line is stopped, these cups may catch fire or melt. Moreover, the line uses guide rails to stabilize the cups within the oven, but because of the extreme heat generated by the open-ended oven, it is necessary to run cooling water through the guide rails to keep the rails from overheating. This requirement adds to the expense and complexity of the machinery. Traditional lip rolling machines induce large amounts of heat into the open oven, and thus, utilize large amounts of energy.
Typically, lip rolling machines guide the cups along water rails up to a lip rolling screw assembly, within the clamshell oven, with the top open and the oven hot. Once the cups engage the screw(s), the oven lid is closed and the rim rolling process begins. As a result, a length of cups equal to or greater than the distance between a crowd roller assembly and the lip rolling screw assembly must be removed and discarded due to the insufficiency of heat applied to the cups from the oven being open and the lack of pressure applied by the crowd rollers.
The present invention is provided to solve these and other such problems with prior art devices.
The present invention provides a method for rolling lips about the open mouths of stacked thermoplastic containers. One particular method comprises the steps of presenting a nested stack of containers having unrolled lips to a rim roller, engaging individual cups on a screw assembly, heating a supply of air in a heater to a desired temperature, discharging the supply of heated air toward the containers as they are engaged on the screw assembly, and rolling the lips of the containers in the screw assembly.
One aspect of the present method further comprises the step of preventing the screw assembly from operating until the supply of air reaches the desired temperature. Additionally, the step of moving the heater (e.g., a forced air heater) to a position proximate the nested stack of containers when the supply of air reaches the desired temperature is an aspect of the invention. The screw assembly includes from one to five screw curlings.
Another aspect of the method further comprises the step of moving the heater to a position distant from the nested stack of containers in the event of a malfunction or stoppage. This may be achieved by actuating a movable cylinder having the heater attached thereto. The step of actuating may be accomplished in one way by sensing a desired temperature at a position proximate the nested stack of containers.
Finally, in another aspect of the present invention, the method further comprises the step of passing the stack of containers under a crowd brush to maintain the nested arrangement of the containers as they enter the screw assembly.
Other features and advantages of the invention will be apparent from the following specification taken in conjunction with the following drawings.